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Pulsar Winds and Jets

Pulsar Winds and Jets. Pat Slane (Harvard-Smithsonian Center for Astrophysics). Outline. Pulsar Winds and Their Nebulae Jet/Torus Structure in PWNe Observations of Pulsar Jets - Jet sizes and luminosities - Curved jets and instabilities

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Pulsar Winds and Jets

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  1. Pulsar Winds and Jets Pat Slane (Harvard-Smithsonian Center for Astrophysics)

  2. Outline • Pulsar Winds and Their Nebulae • Jet/Torus Structure in PWNe • Observations of Pulsar Jets • - Jet sizes and luminosities • - Curved jets and instabilities • - Jet/counterjet asymmetries: Doppler beaming • Geometry from jets: spin-kick alignment

  3. http://www.astroscu.unam.mx/neutrones/home.html

  4. + G + + + + R b Pulsar Wind Nebulae • Expansion boundary condition at • forces wind termination shock at • - wind goes from inside to • at outer boundary } Pulsar logarithmic radial scale • Pulsar wind is confined by pressure • in nebula • - wind termination shock Wind MHD Shock Blast Wave obtain by integrating radio spectrum Particle Flow Ha or ejecta shell • Wind is described by magnetization • parameter • s = ratio of Poynting flux to particle • flux in wind • Pulsar accelerates • particle wind - spectral break at where synchrotron lifetime of particles equals SNR age - radial spectral variation from burn-off of high energy particles • wind inflates bubble • of particles and • magnetic flux • particle flow in B-field • creates synchrotron • nebula

  5. pulsar axis van der Swaluw 2003 Begelman & Li 1992 Elongated Structure of PWNe pulsar axis • Dynamical effects of toroidal field • result in elongation of nebula along • pulsar spin axis • - profile similar for expansion into ISM, • progenitor wind, or ejecta profiles • - details of structure and radio vs. X-ray • depend on injection geometry and B • MHD simulations show B field • variations in interior • - turbulent flow and cooling could result in • additional structure in emission

  6. 3C 58 Slane et al. 2004 G5.4-0.1 Lu et al. 2002 G11.2-0.3 Crab Nebula pulsar spin Roberts et al. 2003 Elongated Structure of PWNe

  7. v ring The Crab Nebula in X-rays Just like the cartoon! Except for all the details… • Emission is dominated by a • bright toroidal structure • - equatorial-dominated outflow • Inner ring of x-ray emission • associated with shock wave • produced by matter rushing • away from neutron star jet - corresponds well with optical wisps delineating termination shock boundary • Curved X-ray jetappears • to extend all the way to the • neutron star • - faint counterjet also seen • - jet axis ~aligned with pulsar proper • motion, as with Vela Pulsar (more on that later…)

  8. Jet/Torus Structure in PWNe • Anisotropic flux with • maximum energy flux • in equatorial zone • - radial particle outflow • - striped wind from • Poynting flux decreases • away from equator • Wind termination • shock is farther from • pulsar at equator • than along axis • Magnetization  is low • in equatorial region • due to dissipation in • striped wind • (reconnection?) • - no collimation along • equator; an equatorial • disk (i.e. torus) forms • At higher latitudes, • average B field is a • maximum • - this can turn the flow • inward at high latitudes, • collimating flow and • forming a jet beyond • TS, where flow is mildly • (or non-) relativistic Lyubarsky 2002

  9. Rotating magnetosphere • generates E X B wind • - direct particle acceleration • as well, yielding • (e.g. Michel 1969; Cheng, • Ho, & Ruderman 1986) The Pulsar Wind Zone • Magnetic polarity in • wind alternates spatially • - magnetically “striped” wind • - does reconnection result in • conversion to kinetic energy? • (e.g. Coroniti 1990, Michel • 1994, Lyubarsky 2002)

  10. Rotating magnetosphere • generates E X B wind • - direct particle acceleration • as well, yielding • (e.g. Michel 1969; Cheng, • Ho, & Ruderman 1986) The Pulsar Wind Zone • Magnetic polarity in • wind alternates spatially • - magnetically “striped” wind • - does reconnection result in • conversion to kinetic energy? • (e.g. Coroniti 1990, Michel • 1994, Lyubarsky 2002) • Wind expands until ram • pressure is balanced by • surrounding nebula • - flow in outer nebula restricts • inner wind flow, forming • pulsar wind termination • shock

  11. PWN Jet/Torus Structure • Poynting flux from outside pulsar light • cylinder is concentrated in equatorial • region due to wound-up B-field • - termination shock radius decreases with • increasing angle from equator (Lyubarsky 2002) • For sufficiently high latitudes, magnetic • stresses can divert particle flow back inward • - collimation into jets may occur • - asymmetric brightness profile from Doppler • beaming • Collimation is subject to kink instabilities • - magnetic loops can be torn off near TS and • expand into PWN (Begelman 1998) • - many pulsar jets are kinked or unstable, • supporting this picture torus pulsar spin axis jet Komissarov & Lyubarsky 2003

  12. PWN Jet/Torus Structure • Poynting flux from outside pulsar light • cylinder is concentrated in equatorial • region due to wound-up B-field • - termination shock radius decreases with • increasing angle from equator (Lyubarsky 2002) • For sufficiently high latitudes, magnetic • stresses can divert particle flow back inward • - collimation into jets may occur • - asymmetric brightness profile from Doppler • beaming • Collimation is subject to kink instabilities • - magnetic loops can be torn off near TS and • expand into PWN (Begelman 1998) • - many pulsar jets are kinked or unstable, • supporting this picture torus pulsar spin axis jet Del Zanna et al. 2006 See poster 53: “Simulated Synchrotron Emission from Pulsar Wind Nebulae (D. Volpi)

  13. PWNe with Anisotropic Axisymmetric Winds Vela Crab PSR B1509-58 Helfand et al. 2001 Weisskopf et al. 2000 Gaensler et al. 2002 • Optical and X-ray studies of Crab have long indicated axisymmetric structure. • - Chandra and HST observations confirm inner ring, torus, jet, and counter-jet • High resolution X-ray image of Vela Pulsar also reveals jets and arc-like features • - For both Crab and Vela, jet is ~along direction of proper motion; geometry related to kicks? • PWN associated with PSR 1509-58 shows complex outflows and arcs • - All suggest equatorial flows and jets from axisymmetric winds

  14. PWNe with Anisotropic Axisymmetric Winds SNR 0540-69 G0.9+0.1 CTA 1 10 arcsec 10 arcsec Gaensler et al. 2002 Gotthelf & Wang 2000 • The closer we look… • - SNR 0540-69 shows a Crab-like nebula surrounding the 60 ms pulsar • - G0.9+0.1 shows a faint point source (pulsar?), a jet axis with arcs, and bright extended • feature that seems to break the symmetry • - CTA 1 shows extended source and jet; no pulsations (yet), but we now know it’s a pulsar… • These axisymmetric structures are now the rule. What are they telling us?

  15. 4’ = 6 pc 40” = 0.4 pc Crab Nebula (Weisskopf et al 2000) PSR B1509-58 (Gaensler et al 2002) Jet Sizes and Luminosities • Jets are observed for ~8-12 young • pulsars • - the more we look the more we find, • though evidence is weak for some • Sizes vary from <0.1 pc (CTA 1) to • >10 pc (PSR B1509-58) • - no strong connection with Edot • Jet luminosity ranges from • Typical photon index  ~ 1.5 - 2 • - generally, uncooled synch. spectrum • Where known, outflow velocities • are subsonic 13” = 0.2 pc 80” = 0.8 pc Vela Pulsar (Pavlov et al. 2003) 3C 58 (Slane et al. 2004)

  16. Jet Sizes and Luminosities • Jets are observed for ~8-12 young • pulsars • - the more we look the more we find, • though evidence is weak for some • Sizes vary from <0.1 pc (CTA 1) to • >10 pc (PSR B1509-58) • - no strong connection with Edot • Jet luminosity ranges from • Typical photon index  ~ 1.5 - 2 • - generally, uncooled synch. spectrum • Where known, outflow velocities • are subsonic Warning: Better estimates of geometry/beaming needed for these!

  17. Curved Jets and Instabilities PSR 1509-58 Pavlov et al. 2003 • Jet in PSR 1509-58 is curved, much like in Crab • - variations in structure seen on timescale of • several months (v ~ 0.5c) • Jet in Vela is wildly unstable, showing variations • on timescales of weeks to months • - changes in morphology suggest kink or sausage • instabilities DeLaney et al. 2006

  18. Curved Jets and Instabilities PSR 1509-58 Pavlov et al. 2003 • Jet in PSR 1509-58 is curved, much like in Crab • - variations in structure seen on timescale of • several months (v ~ 0.5c) • Jet in Vela is wildly unstable, showing variations • on timescales of weeks to months • - changes in morphology suggest kink or sausage • instabilities DeLaney et al. 2006

  19. 13” = 0.2 pc Slane et al. (2004) Curved Jets and Instabilities 3C 58 • Pulsar in 3C 58 has curved jet-like structure • - jet is opposite bright torus edge; faint counterjet seen • Nebula shows complex of loop-like filaments, apparently • originating near pulsar • - particularly evident in higher energy X-rays; not seen in optical • These do not appear similar to Crab filaments, which are • from R-T instabilities as PWN expands into ejecta • - are 3C 58 structures loops of magnetic flux torn from • jet region due to kink instabilities (e.g. Begelman 1998)? Crab Nebula

  20. Jet/Counterjet Asymmetries: Doppler Beaming • For several pulsars, both a jet and a • counterjet are observed. • - for most, the flux from the counterjet is difficult to • measure, both because it is faint and because the • surrounding PWN emission is bright • The expected intensity ratio from Doppler • beaming is • Using inclination angles determined from • the torus, along with measured spectrum, • predictions can be made for  • - these can be compared with measured flow • flow speeds, where available (e.g. Crab, Vela) • - agreement for Crab and Vela is reasonable, but • 3C 58 predicts a very large value, though the • uncertainties in flux ratio and angle are large • - for PSR 1706-44, the counterjet seems to be • on the wrong side (Romani et al. 2005)!

  21. Spin-Kick Alignments? Romani & Ng 2003 • Assuming jet-toroid geometry, spatial • modeling can reveal the geometry of • PWN systems • Ex: PSR J0538+2817 • - Chandra image shows point source • surrounded by extended emission • - image modeling suggests a tilted torus • surrounding a NS; torus angle can be • estimated by spatial modeling • Pulsar is located in SNR S147 • - offset from center gives kick velocity • - kick appears to be aligned with spin axis • Initial spin period ~130 ms (slow…) • - inferred v > 130 km/s; spin-kick alignment • constrains models; kick mechanism needs to • average over initial spin period • - EM kick requires initial P < 3 ms; hydro kicks • too fast as well; may need asymmetric • n emission to explain alignment

  22. v ring Recent work by Ng & Romani (2006) shows that proper motion is misaligned from jet axis by ~26 degrees for Crab. This suggests rotational averaging of kick did not occur, suggesting kick timescale < 20 ms. The Crab Nebula in X-rays How does pulsar energize synchrotron nebula? Pulsar: P = 33 ms dE/dt = 4.5 x 10 erg/s 38 37 Nebula:L = 2.5 x 10 erg/s x • X-ray jet-like structure appears • to extend all the way to the • neutron star • - jet axis ~aligned with pulsar proper • motion; same is true of Vela pulsar jet • inner ring of x-ray emission • associated with shock wave • produced by matter rushing • away from neutron star - corresponds well with optical wisps delineating termination shock boundary

  23. TeV -rays from PWNe • Particles are accelerated to high • energies in PWNe • - for Crab Nebula, inverse-Compton • scattering of synchrotron photons • produces TeV gamma-rays • For lower magnetic field objects, • synchrotron-emitting particles • radiating in a given band are • more energetic than for Crab • - these can produce TeV gamma-rays • by IC-scattering of CMB photons • For fields in PSR B1509-58, • electrons producing TeV -rays • would radiate synchrotron photons • in UV • - for PWNe, HESS/Veritas are UV • telescopes!

  24. TeV -rays from PWNe • Particles are accelerated to high • energies in PWNe • - for Crab Nebula, inverse-Compton • scattering of synchrotron photons • produces TeV gamma-rays • For lower magnetic field objects, • synchrotron-emitting particles • radiating in a given band are • more energetic than for Crab • - these can produce TeV gamma-rays • by IC-scattering of CMB photons • For fields in PSR B1509-58, • electrons producing TeV -rays • would radiate synchrotron photons • in UV • - for PWNe, HESS/Veritas are UV • telescopes! Aharonian et al. 2005 6 arcmin

  25. Conclusions • For “standard” young pulsars, we expect pulsar wind nebulae • - properties of nebulae point to properties of neutron star • - high resolution X-ray studies reveal neutron stars, • termination shock from pulsar wind, equatorial and axial • structures (i.e. jets) • Jets are often curved and show variable structure • - kink instabilities in collimated flow? • Counterjets are faint and can be difficult to detect against PWN • - consistent w/ Doppler beaming? probably, but hard to get • good measurements of flux ratio and flow speeds • Jet/Torus geometry defines spin axis. Evidence for spin-kick • alignment? • - jury is out, but it is looking less likely that this is the case • - suggests fast kick mechanism • TeV -rays from pulsar jets observed; new constraints on particle spectrum

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